1. Field of the Invention
The invention relates in general to a semiconductor device, a data element and a method of fabricating the same, and more particularly to a programmable read-only memory device and a method of fabricating and operating the same.
2. Description of the Related Art
Semiconductor nonvolatile memories may be divided into two categories: (1) those in which data is permanently written during the manufacturing process and whose contents cannot be subsequently changed, called “mask ROMs” or “factory programmed ROMs”; (2) those in which data may be supplied after the finished memory device leaves the factory. This latter category is called “field programmable memories” because their contents may be written, by the user, when the semiconductor memory chip is deployed to its final application, “in the field”.
Field programmable memories are further subdivided into “write once” memories and “write/erase/rewrite” memories. Those written once are referred to as “PROM” (programmable read-only memories) or “OTP ROM” (one time programmable read only memories). And those memories that provide write/erase/rewrite capabilities have been referred to as “UVEPROM” (ultraviolet erasable programmable read only memories) or “EEPROM” (electrically erasable programmable read only memories) or “Flash EEPROM” (fast and flexible EEPROMs). In contrast, the contents of mask ROMs are permanently stored during manufacture, therefore mask ROMs are not erasable and are effectively “write only once, at the factory” memories.
Field programmable memories are much more flexible than mask ROMs, since they allow system product makers to inventory a single general part-type for many applications, and to personalize (program the memory contents of) this one part-type in numerous different ways, much later in the system product flow. This flexibility lets system manufacturers more easily adapt to fluctuations in demand among different system products, and to update or revise system products without the expense of scrapping (discarding) existing inventories of pre-programmed mask ROMs. But this flexibility has a cost: field programmable memories generally achieve lower densities (fewer bits per chip) and higher cost (larger price per bit) than mask ROMs.
A well-known “OTP ROM” is produced by Matrix Semiconductor Inc. One layer of a layer stack constituting the OTP ROM is made of silicon dioxide. A silicon coating is oxidized to the silicon dioxide by thermal oxidation process. However, most thermal oxidation is performed in furnaces, at temperatures between 800 and 1200° C. The memory device would be deteriorated or damaged under such high temperature. Besides that, the finished OTP ROM must be programmed by applying a high-voltage pulse for a relatively long time, such as applying at least 10 Volt for 2 microseconds. The memory can be programmed just once after manufacturing by “blowing” the fuses (using a PROM blower), which is an irreversible process.
There is a need, therefore, for a programmable read-only memory device being capable of readily fabricated and reliably programmed.